Tolerance to kidney allografts has been achieved in nonhuman primates (NHPs) and, for the first time, in humans using a combination of nonmyeloablative conditioning and donor bone marrow transplantation that results in transient donor chimerism. However, mixed chimerism protocols that induce long term tolerance to kidney allografts in NHPs fail to do so in recipients of cardiac allografts. The goal of Project 1 is to use new agents and novel strategies to optimize mixed chimerism so that it will induce tolerance to cardiac allografts and prevent cardiac allograft vasculopathy (CAV). We hypothesize that 1) targeting newly recognized barriers to tolerance will allow us to alter the balance of an alloresponse away from alloaggression and towards deletion/regulation leading to long-term tolerance, 2) humoral responses and proinflammatory states are particularly detrimental to tolerance induction in cardiac allograft recipients and may be important confounding factors limiting the duration of the state of mixed chimerism, and 3) durable mixed chimerism will likely be required for tolerance in heart recipients because, unlike kidneys, cardiac allografts do not appear capable of maintaining tolerance once chimerism disappears. In the specific aims of each Project, the contributions of allo-, auto- and natural antibodies to graft loss after mixed chimerism induction are recognized and addressed in an innovative and complementary manner. Likewise, the deleterious effects of proinflammatory molecules are considered throughout the program and addressed with innovative and complementary approaches. We anticipate ongoing progress will continue to contribute to a reduction in the morbidity and mortality associated with solid organ transplantation. RELEVANCE (Seeinstructions): The survival of human recipients of heart transplants is limited by rejection and by the complications of anti- rejection drugs. The objective of this proposal is to induce a state of immune tolerance in the recipient, which tricks the immune system into seeing the foreign graft as its own tissue, so that the organ is not rejected without the need for drugs. This would greatly improve the outcomes of heart transplants.